1. Field of the Invention
This invention relates to a rear focus type zoom lens, and particularly to a compact rear focus type zoom lens suitable for use in a video still camera or the like, in which the zoom ratio has a high variable power ratio, and yet the number of constituent lenses is small, the aperture ratio is great and the mechanism is simple.
2. Related Background Art
In zoom lenses used in photographic cameras, video cameras, etc., there have heretofore been various examples adopting the so-called rear-focus-type system in which lens units rearward of the first lens unit on the object side are moved to effect focusing. This is because the rear-focus-type system moves relatively compact and light-weight lens units, and therefore, the driving force for the lens units may be small and quick focusing is possible, and therefore the rear-focus-type system is compatible with an auto-focus system.
For example, in Japanese Patent Application Laid-Open Nos. 62-206516, 62-24213, 62-247316 and 4-43311, the zoom lens has four lens units in succession from the object side, i.e., a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, and the second lens unit is moved to effect a focal length change, and the fourth lens unit is moved to correct the fluctuation of the image plane resulting from the focal length change and also to effect focusing.
Japanese Patent Application Laid-Open No. 63-29718 discloses a zoom lens comprising, in succession from the object side, a first lens unit of positive refractive power, a second lens unit comprised of three lenses, i.e., a negative lens, a negative lens and a positive lens, having negative refractive power as a whole, movable during a focal length change and governing chiefly the focal length change, a third lens unit having positive refractive power and including an aspherical surface, and a fourth lens unit having positive refractive power with a somewhat great air space with respect to the third lens unit and correcting the fluctuation of the image plane resulting from the focal length change and movable for focusing.
Japanese Patent Application Laid-Open No. 5-72474 discloses an aspherical zoom lens having, in succession from the object side, a fixed first lens unit having positive refractive power, a second lens unit for focal length change having negative refractive power, a fixed third lens unit of positive refractive power having a condensing action, and a fourth lens unit of positive refractive power movable on an optical axis to maintain the position of the image plane. In this zoom lens, the second lens unit comprises a meniscus negative lens, a biconcave lens and a positive lens, the third lens unit is comprised of a single lens having one or more aspherical surfaces, and the fourth lens unit is comprised of a lens having one or more aspherical surfaces.
U.S. Pat. No. 4,299,454 discloses a zoom lens comprising, in succession from the object side, a first positive lens unit, a second negative lens unit, and a rearward positive lens unit, at least two lens units including the negative lens unit being moved to effect a focal length changes, the second negative lens unit comprising, in succession from the object side, first and second negative lenses and a positive doublet.
In the so-called four-unit rear focus zoom lens, there is a method of strengthening the refractive power of the second lens unit for focal length change and reducing the amount of movement of the second lens unit for securing a predetermined variable power ratio in the direction of the optical axis thereof.
When the construction as described above is adopted, the spacing between the first lens unit and the second lens unit, which are a focal length changing system, becomes short, and the distance from a stop to the first lens unit becomes short and therefore, the diameter of the fore lens becomes small. As a result, it becomes possible to make the thickness of the first lens unit small and therefore, the downsizing of the entire lens system becomes easy.
As a downsizing method, there is a method of decreasing the number of lenses. Specifically, there is a method of constructing the third lens unit of a lens and constructing the fourth lens unit of three or less lenses to achieve downsizing.
Zoom lenses of such construction have been proposed, for example, in Japanese Patent Application Laid-Open Nos. 62-178917, 64-68709, 2-53017, 4-78807, 5-60972, 5-72474, 6-347697, 7-113955, 7-199069, 7-261081, 8-5916, 8-5917, 8-179206, 8-292369, 9-211324, 11-84238, etc.
It is an object of the present invention to provide a rear focus type zoom lens which enables the downsizing of the entire lens system, quick focusing and the simplification of a mechanical mechanism to be achieved and in which the zoom ratio is a high variable power ratio and yet the number of constituent lenses can be reduced under a great aperture ratio and which can be suitably used particularly in a video still camera or the like.
It is another object of the present invention to provide a rear focus type zoom lens having, in succession from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein the third lens unit has a stop most adjacent to the image plane side, the surface thereof adjacent to the stop is convex toward the image plane side and aspherical, and when the focal lengths of the entire system at the wide angle end and the telephoto end are defined as fw and ft, respectively, and the F number of the wide angle end is defined as FNW, and the focal length of the second lens unit is defined as f2, the following conditional expression is satisfied:
0.8 less than {|f2/fA|}xc3x97FNW less than 1.6
(fA={square root over (fwxc2x7ft)})
It is another object of the present invention to provide an optical apparatus having the afore-described rear focus type zoom lens.
It is another object of the present invention to provide a rear focus type zoom lens having, in succession from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein the third lens unit has a stop most adjacent to the image plane side, the surface thereof adjacent to the stop is convex toward the image plane side and aspherical, that surface of the fourth lens unit which is most adjacent to the object side is concave toward the object side, and when the radius of curvature of that surface of the fourth lens unit which is most adjacent to the object side is defined as R4F, and the focal length of the fourth lens unit is defined as f4, the following conditional expression is satisfied:
0.8 less than |R4F/f4| less than 1.8
It is another object of the present invention to provide an optical apparatus having the above-described rear focus type zoom lens.
It is another object of the present invention to provide a rear focus type zoom lens having, in succession from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein the third lens unit has a stop most adjacent to the image plane side, the surface thereof adjacent to the stop is convex toward the image plane side and aspherical, and when the radius of curvature of that surface of the second lens unit which is most adjacent to the object side is defined as R2F and the focal length of the second lens unit is defined as f2, the following conditional expression is satisfied;
11 less than |R2F/f2| less than 84
It is another object of the present invention to provide an optical apparatus having the above-described rear focus type zoom lens.
It is another object of the present invention to provide a rear focus type zoom lens having, in succession from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein the third lens unit has a stop most adjacent to the image plane side, the surface thereof adjacent to the stop is convex toward the image plane side and aspherical, and when the refractive index of the medium of the negative lens of the second lens unit which is most adjacent to the object side is defined as N2f, the following conditional expression is satisfied:
1.83 less than N2f less than 1.91
It is another object of the present invention to provide an optical apparatus having the above-described rear focus type zoom lens.
It is another object of the present invention to provide a rear focus type zoom lens having, in succession from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, wherein the third lens unit has a stop most adjacent to the image plane side, the surface thereof adjacent to the stop is convex toward the image plane side and aspherical, and the following conditional expression is satisfied:
0.05 less than (D3wxe2x88x92D3T)/fA less than 0.14,
where D3w represents the distance between the third lens unit and the fourth lens unit when focusing on an infinity object upon the wide angle end and D3T represents the distance between the third lens unit and the fourth lens unit when focusing on an infinity object upon the telephoto end.
It is another object of the present invention to provide an optical apparatus having the above-described rear focus type zoom lens.
It is another object of the present invention to provide a rear focus type zoom lens having, in succession from the object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, the second lens unit being moved to the image plane side during the focal length change from the wide angle end to the telephoto end, the fourth lens unit being moved when the correction of the fluctuation of the image plane resulting from the focal length change is to be effected, wherein the third lens unit has a stop most adjacent to the image plane side, the surface thereof adjacent to the stop is convex toward the image plane side and aspherical, and when the refractive index of the medium of the positive lens of the first lens unit which is most adjacent to the image plane is defined as N1r, the following conditional expression is satisfied:
1.75 less than N1r less than 1.91
It is another object of the present invention to provide an optical apparatus having the above-described rear focus type zoom lens.
Other objects and features of the present invention will become apparent from the following detailed description of the invention taken in conjunction with the accompanying drawings.